One traditional approach to simulating electrical circuits is to use a version of the SPICE (Simulation Program with Integrated Circuit Emphasis) transistor level circuit simulator, originally created by the University of California, Berkeley in 1972, or one of its commercial variants such as SPECTRE™ by Cadence Design Systems, Inc., HSPICE™ by Synopsys, Inc., or ELDO™ by Mentor Graphics, Corp.
Referring now to FIG. 1, SPICE transistor level circuit simulators are well suited for simulating circuits that process lower frequency analog signals, such as illustrated by waveform 101 in FIG. 1. An analog signal is an electronic signal whose value varies continuously with time. However, SPICE transistor level circuit simulators are slow when simulating a circuit with modulated RF signals such as the waveform 102 illustrated in FIG. 1. RF signals are higher frequency analog signals and may be formed by modulating a low frequency analog signal with a repetitive high frequency modulating signal. Typically the low frequency analog signal modulates the magnitude, the phase, or the frequency of the repetitive high frequency signal.
SPICE transistor level circuit simulators are also relatively slow in simulating digital circuits. Waveform 103 in FIG. 1 illustrates an exemplary digital signal having a rectangular or square wave shape. Logic simulators are often preferred over SPICE transistor level circuit simulators for digital circuits. Logic simulators simulate digital circuits with digital signals faster than SPICE transistor level circuit simulators can because they represent the circuit at a higher level of abstraction and use a very fast simulation algorithm that is referred to as event driven simulation. The most popular logic simulators in use today for digital signals are either Verilog or VHDL simulators, both or which are very similar.